As antenna systems grow smaller, space between antenna elements in those systems becomes more scarce. Not only does the spacing between antenna elements have the potential to affect the radiation pattern of a system, but it can also affect the amount of mutual coupling between antenna elements. Mutual coupling is inductive/capacitive coupling between two or more antennas, and it can sometimes result in unwanted performance degradation by interfering with signals being transmitted or by causing an antenna element to radiate unwanted signals. Generally, the closer the placement of two antenna elements, the higher the potential for mutual coupling.
Accordingly, modern antenna designers generally look for ways to decrease coupling (i.e., increase isolation) between some antenna elements. This is especially true for multi-channel systems, as the signals on one channel should usually and ideally be unaffected by the signals on other channels. It is also particularly true for Multiple Input Multiple Output (MIMO) antenna systems which require several antennas to operate at the same frequency but work independently of each other.
Some antenna systems employ antenna elements placed above a ground plane. In such systems, the antenna elements can induce currents in the ground plane that travel to other antenna elements and increase undesired coupling. To decrease the coupling, various techniques have been devised. For example, one solution has been to split the ground plane so that two antennas that might interfere are not connected by a continuous ground plane. However, such systems generally produce an inadequate amount of isolation.
Other proposed systems include intricate fabrication processes to produce structures with cells shorted to the ground through vias in a Printed Circuit Board (PCB). Such structures are analogous to Photonic Band Gap (PBG, used in optics) structures and generally act as bandstop filters and can be designed to cancel specific, unwanted signals. However, such systems are expensive in terms of both space and money because of the complexity of the three-dimensional shapes of the structures. Currently, no prior art system provides adequate isolation with a minimum of complexity.